When an article of manufacture is created, processes such as casting, forging, etc., are used to give the material the desired shape with the sought bulk mechanical properties for the specific application. However, in many applications, the surface of the object is exposed to diverse harsh environments such as abrasive, corrosive and high temperature environments, to name a few. Those environments can degrade the surface of the object and its properties, eventually leading to its failure. Thermal spray (TS) processes are used to deposit coatings, from a few microns to a few millimeters thick, to prevent the degradation of the coated surface. TS technology is used by an increasing number of manufacturers to produce high-quality competitive products. TS encompasses a wide variety of processes that often have a common purpose: to modify the surface properties of existing objects to increase their performances and/or lifetimes. Alternatively, TS processes can allow material deposition to generate objects having a specific shape or form.
Typically, TS processes have in common that a feedstock material in powder, wire or rod form is heated to a molten or semi-molten droplet state that is preferably accelerated onto the surface to be coated. Upon impact, the particles deform, adhere to the substrate and solidify (if they were molten) building a lamellar structure to form the desired coating. The heat source to heat up or melt the feedstock particles can, for example, be a flame (resulting from the combustion of fuels) or electric arc (resulting form gas ionization). The particles are accelerated by a flow of the heated gas towards the substrate. Complete coatings may be achieved by moving the spray apparatus or the substrate relative to each other and a number of spray passes may attain the desired coating thickness.
TS processes may be used to modify or enhance the surface properties of an extensive variety of objects/surfaces of various materials by applying metallic, alloys, ceramics, polymers, cermets or carbides coatings upon them. TS coatings are used in a broad variety of industrial sectors and products such as gas and steam turbines, automotive engines, iron and steel manufactures and mills, ship and boat manufactures and repairs, chemical processing plants, electrical utilities, pulp and paper sector, defense and aerospace devices, food processing plants and mining, to name a few.
The coatings applied to the different substrates are generally grouped according to their function. Some important coating functions are: wear resistance, chemical resistance, provide thermal insulation, corrosion resistance, electrical conductivity or resistance, biocompatibility, radiative shielding, abrasive and purely cosmetic, to name a few. A coating can provide more than one function if required.
Particle temperature and velocity prior to impact is an important parameter combination determining the coating quality. Historically, TS processes have evolved towards higher particle impact velocities as they generally lead to denser coatings with improved bond strength and reduced residual stress. Previously, this has been accomplished by accelerating the propellant gas/mixture through a converging-diverging nozzle to reach supersonic velocities, to increase the propellant/particle momentum transfer. However, high particle velocities can become detrimental when the particles are fully molten prior to impact. In that case, the force exerted on the molten particle can be so large that it leads to particle breakup and/or splashing of the particles upon impact. The resulting coatings are not as dense and do not exhibit as strong bond strength. Consequently, it is customary to reduce the particle temperature as the particle velocity increases to avoid this phenomenon.
The chemical and microstructural composition of the particles prior to impact is also an important parameter affecting the coating properties and quality. Most existing TS processes lack control of the chemical composition and microstructure of the particles prior to impact due to the highly reactive propellant gas mixture into which the particles are injected to be accelerated, and optionally heated. This leads to oxidation of the particles, changes in their microstructure and/or chemical composition. Consequently, it is difficult to predict the coating chemical composition and microstructure and to tailor the feedstock material based on the required coating properties. For the same reasons, producing nanocrystalline coatings is a challenge using TS processes due to the grain growth encountered in the coating due to the heating of the particles and coatings.
Despite the widespread use of TS coatings in all industrial sectors, there is a constant demand from the manufacturers to produce higher performance and longer lasting TS coatings and objects.